Principal Cathy O'Donnell -- who was out sick last week -- said Thursday evening that the virus strikes quickly, making it hard to keep the children home before they get sick.

"You're fine, then all of a sudden, you're not," O'Donnell said.

Illness at Black Rock Elementary

An unidentified viral illness that causes nausea, vomiting and fever is rapidly spreading among students and staff.

If you are ill: Notify the doctor or emergency room staff that the Weld County Department of Public Health and Environment is asking for viral and bacterial stool cultures as it investigates the cause of the illness.

Precautions:

Wash hands thoroughly after using the restroom and before preparing or serving food.

Although the first signs of illness showed up about two and a half weeks ago, the number of absences wasn't a big concern until Wednesday, she said.

By the end of Thursday, 97 students were absent, she said. One first-grade class of 22 had only six students left by the end of the day, she said.

"It's a tricky one because it comes on so quickly," O'Donnell said.

School district spokeswoman Shari Wilson said five staff members also have been ill in the past couple of days.

"Several families have had it," O'Donnell said.

Health department officials suggested that some students may have returned to school while they were still contagious, spreading the virus, she said.

She urged parents to keep their children at home for at least 24 hours after symptoms have ended. Most people need to be out for two or three days, she said.

As a precaution, the school will be thoroughly cleaned over the weekend, according to St. Vrain Valley School District officials. It is scheduled to reopen on Monday.

When absences rose Wednesday, the district's nurse contacted the Weld County Department of Public Health and Environment, O'Donnell said. The decision to close the school was made late Thursday afternoon.